1. Field of the Invention
The present invention relates to an image forming apparatus, and more particularly to an image forming apparatus that emits light from a light source, that deflects the light with a deflector and that scans a photoreceptor with the light.
2. Description of Related Art
An example of conventional image forming apparatuses is a color image forming apparatus disclosed by Japanese Patent Laid-Open Publication No. 2009-244843. In the color image forming apparatus, the light intensity of a laser beam is adjusted in the way described below. FIGS. 11a to 11d are signal waveform diagrams used for the light intensity adjustment to the laser beam in the color image forming apparatus. FIG. 11a shows an output waveform from a PD (photodiode), and FIG. 11b is an output waveform from a COMP circuit. FIG. 11c shows an output waveform from a delay circuit, and FIG. 11d shows a waveform of a light intensity monitor signal.
When the PD receives light, the PD generates a PD output as shown by FIG. 11a. The COMP circuit generates a COMP output as shown by FIG. 11b, and the COMP output is on a high level while the PD output is over a threshold value. The delay circuit generates a delay output that rises to a high level with a delay of a specified time from the rise of the COMP output. The rise of the delay output incidents with the peak (the center of the high-level period) of the PD output. As shown by FIG. 11d, the light intensity monitor signal is generated based on the delay signal. The light intensity monitor signal is used for the light intensity adjustment to the laser beam.
In the color image forming apparatus disclosed by Japanese Patent Laid-Open Publication No. 2009-244843, however, the rise of the delay output may not be incident with the peak (the center of the high-level period) of the PD output. More specifically, the high-level potential of the PD output fluctuates. When the high-level potential of the PD output is relatively low, the PD output rises gradually, and when the high-level potential is relatively high, the PD output rises steeply. Accordingly, the time when the PD output comes over the threshold value depends on the value of the high-level potential of the PD output. The delay time from the COMP output is fixed although the high-level potential of the PD output fluctuates. Therefore, the rise of the delay output may not be incident with the peak (the center of the high-level period) of the PD output.